They have already delivered this support to the dev channel (version 6)
of Chrome and I'm running it. As Google implies in their announcement,
the PDF rendering support is sort-of early, but fascinating nonetheless.
For an interesting sample, try the IRS form W-9 which
includes fillable forms. The forms are dead in the current version of
the Google PDF plugin, but on the other hand you can see their work in
attempting to make PDFs render more like HTML. The text in the document
is selectable, for example.

From a security standpoint Chrome's native PDF support is interesting in at least 3 ways: First, it's not Adobe Reader or Acrobat, so exploits of vulnerabilities in those programs won't likely work in the Chrome native code. But there have been alternative PDF readers for a long time, so in that sense this is just one more, and it only works in Google's browser.

The better reason to use it is that the plugin runs in the Chromium sandbox, so any vulnerabilities in it are likely not exploitable in any meaningful way. I've argued since it came out that Chrome's security architecture is the best available.

The final reason to use it is that, like Chrome's integrated Adobe Flash client, security updates to Chrome's native PDF reader will be sent automatically and updates will happen quickly, Old versions of Adobe Reader, on the other hand, are the Internet's vast playground of the malicious. Note that this PDF support is unlike the Flash support in that the Flash code is Adobe's and just integrated and distributed by Google. The PDF code is written by Google to the PDF spec.

The reason not to use it is the flip side of the first reason to use it: it's not Adobe Reader or Acrobat, so PDF authors won't likely make sure that their documents render properly in Google's native PDF reader. What version of the PDF spec will they commit to support? Will they support all features? These issues still have to be ironed out.